The broad long-term goal of this project is to elucidate the molecular basis of environmental Polycyclic Aryl-Hydrocarbon (PAH)-induced cell cycle checkpoint control. We have identified a novel cell cycle checkpoint resulting from aryl-hydrocarbon-induced DNA damage. The PAH- activated checkpoint arrests proliferating cells in the G1 phase of the cell cycle. Growth arrest prior to S-phase prevents potentially error- prone and mutagenic replication of damaged DNA. Experiments proposed here seek to understand the molecular mechanism(s) whereby the PAH- induced checkpoint is imposed upon cells and, ultimately, how this mechanism is escaped during transformation. Our preliminary studies suggested new and important roles for tumor suppressors (Retinoblastoma, or Rb, and p53) and G1 signal transduction pathways in PAH-induced checkpoint control. The Rb and p53 genes play important roles in guarding against malignancy and are mutated or absent in many human cancers. Therefore, our preliminary data provided a new and direct link between the cellular effects of environmental PAHs and human malignancies. These studies will test the putative roles of Rb, G1 mitogenic signal transduction events, and p53 in regulating cellular responses to PAHs. The specific aims of this project are: (1) To test the hypothesis that the PAH-induced cell cycle checkpoint is Rb- mediated. (2) To test the hypothesis that the PAH-induced checkpoint results from modification of mitogenic G1 signal transduction events. (3) To test the hypothesis that p53 plays a role in repair of PAH- adducted DNA. We will use loss-of-function (expression of viral oncogenes) and gain-of-function (ectopic expression of tumor suppressors in knockout tumor suppressor-deficient cells) strategies to test the roles of p53 and Rb in cellular responses to PAHs. Additionally, we will analyze the PAH-sensitivity of mitogenic signaling cascades during G1 in order to identify the putative PAH-induced lesion(s). These studies will identify novel molecular mechanisms of checkpoint control. This information may enable the rational design of new chemotherapies to manipulate checkpoint pathways. Such drugs could help prevent and treat PAH-induced (and possibly other) malignancies.